This invention relates to an improved exhaust gas oxygen sensor of the type adapted for installation in a conduit for conveying exhaust gases from an internal combustion engine. The improved sensor is responsive to the partial pressure of oxygen in the exhaust gases to which the sensor is exposed and has an electrical characteristic which varies, when the sensor is at operating temperatures in the range from 350.degree. C. to about 850.degree. C., with the partial pressure of oxygen in the exhaust gases. The invention particularly relates to an improved ceramic insulator used in an exhaust gas sensor of the type employing a metal oxide ceramic material, such as titania, as the oxygen sensing element having precious-metal electrode lead wires passing through passages formed in the ceramic insulator material.
Exhaust gas sensors of the type to which the present invention relates have been fabricated with a titania oxygen sensing element of disc shape having electrode lead wires embedded therein. The lead wires convey to an electronic control system an electrical signal that results from variations in the electrical resistance of the titania disc. The titania resistance changes are produced by corresponding variations in the composition of the exhaust gases to which the sensor is exposed. The electrical lead wires and the titania oxygen sensing element to which they are connected are supported by a ceramic insulator mounted within a steel body that is attached to an exhaust conduit of the engine. The varying electrical signal generated by the titania oxygen sensing element are transmitted by its precious metal electrode lead wires to other lead wires in the ceramic insulator. These other lead wires are connected to terminal pins located at the end of the ceramic insulator.
It has been found that bending of the precious metal oxygen sensing element electrode lead wires, comprised of platinum as the major constituent, produces cold-working of the lead-wire metal that can cause the effected areas to fail by fracture after use in an exhaust gas environment. Failure of the lead wires of the sensor renders it useless.